1. Field of Invention
The present invention relates to a switching regulator compatible with an electronic transformer and a control method thereof; particularly, it relates to such switching regulator and control method thereof wherein an input current is related to a difference between an output voltage and a rectified input voltage.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art switching regulator 100 and an electronic rectifier circuit 110. As shown in FIG. 1A, the switching regulator 100 includes a power stage circuit 10, a control circuit 20, and a feedback circuit 30. The power stage circuit 10 receives a rectified input voltage Vin, and converts the rectified input voltage Vin to an output voltage Vout according to an operation signal GATE. The rectified input voltage Vin is generated by the electronic rectifier circuit 110. The electronic rectifier circuit 110 includes an electronic transformer 40 and a rectifier circuit 50. The electronic transformer 40 receives an AC voltage VAC and converts it to an AC voltage VAC′ which has a lower amplitude. The rectifier circuit 50 receives the AC voltage VAC′, and rectifies it to generate the rectified voltage Vin. The feedback circuit 30 generates a feedback signal FB according to the output voltage Vout. The control circuit 20 is connected to the feedback circuit 30 to receive the feedback signal FB, and it generates the operation signal GATE according to the feedback signal FB to operate a power switch (not shown, referring to FIGS. 2A-2K) in the power stage circuit 10, so as to convert the rectified input voltage Vin to the output voltage Vout, and to control an input current Iin. The power stage circuit 10 may be a synchronous or asynchronous buck, boost, inverting, buck-boost, inverting-boost, or flyback power stage circuit as shown in FIGS. 2A-2K.
The reason that the electronic rectifier circuit 110 uses the electronic transformer 40 is for decreasing the size of the entire circuitry, such that the switching regulator 100 may be applied in an electronic product such as a wall lamp or a downlight. The electronic transformer 40 converts the AC voltage VAC having a higher amplitude, such as 110V or 220V, to the AC voltage VAC′ having a lower amplitude, such as 12V or 14V. The electronic transformer 40 is well known by those skilled in the art, so details thereof are omitted here.
FIG. 1B is a schematic diagram showing signal waveforms of the AC voltages VAC and VAC′, the rectified voltage Vin, and the output voltage Vout. In FIG. 1B, the thin solid lines indicate a zero level or a ground level, and the thick solid lines indicate the signal waveforms of the aforementioned signals. As shown in the figure, the AC voltage VAC has a relatively higher amplitude (the amplitudes in the figure are not shown in actual scale). The electronic transformer 40 oscillates with a frequency higher than the AC voltage VAC, and generates the AC voltage VAC′ as shown in FIG. 1B. For example, the frequency of the AC voltage VAC is 60 Hz, and the electronic transformer 40 oscillates with a frequency for example of 30 kHz in one cycle period, to generate the AC voltage VAC′ having a contour as shown which has a frequency of 120 Hz. The AC voltage VAC′ is rectified to the semi-sinusoidal rectified voltage Vin having a frequency of 120 Hz. The output voltage Vout for example has a fixed level.
In the aforementioned prior art switching regulator 100, the power conversion is achieved by switching one or more power switches in power stage circuit 10, and the switching of the power switch determines the input current Iin. To achieve power factor correction (PFC) for better power factor, in this prior art, a typical method is to control the switching of the power switch such that the peaks (Iin_max) of the input current form a contour which is substantially in phase with the rectified input voltage Vin, as indicated by FIG. 1C. However, as a basic requirement of the electronic transformer 40, the peaks of its load current (the input current Iin is the load current of the electronic transformer 40) must be higher than a minimum load current Iload_min, otherwise the electronic transformer 40 does not operate. In this prior art, because it is uncertain as to what kind of electronic transformer 40 and what kind of switching regulator 100 a user might use, and different electronic transformers 40 have different specifications, the peaks (Iin_max) of the input current Iin might be lower than the minimum load current Iload_min, and in some extreme cases, the electronic transformer 40 may not be able to operate in an entire cycle period of the rectified input voltage Vin, resulting in poor compatibility of the switching regulator 100 and the electronic rectifier circuit 110.
In view of above, the present invention proposes a switching regulator compatible with an electronic transformer and a control method thereof.